1. Field of the Invention
The present invention relates to a process for the alkylation of aromatic compounds by the reaction of the aromatic compound of interest with isopropanol (IPA), alone or mixed with propylene, said reaction taking place in the presence of a catalytic composition based on zeolite and characterized in that the reaction mixture is present in mixed gas-liquid phase or in completely liquid phase under such temperature and pressure conditions that the concentration of water present in the liquid phase does not exceed 8,000 ppm w/w.
In particular, the present invention relates to a process for the alkylation of aromatic compounds in which said alkylation reaction, carried out with isopropanol or mixtures of isopropanol with propylene, is effected in the presence of a catalytic system comprising or consisting of a beta zeolite.
Even more specifically, the present invention relates to a process for the alkylation of benzene with isopropylic alcohol, and optionally propylene, to give cumene in the presence of a catalyst comprising or consisting of beta zeolite, said process being characterized in that the reaction mixture is present in mixed gas-liquid phase or in completely liquid phase under such temperature and pressure conditions that the concentration of water present in the liquid phase of said reaction mixture does not exceed 8,000 ppm w/w, and is preferably lower than or equal to 7,000 ppm w/w.
The above limit is regardless of the total water content in the reaction mixture stoichiometrically corresponding to the total quantity of isopropanol (IPA) in the feeding to the reaction section.
The process is characterized by the total absence of negative effects on the performance and duration of the catalyst due to the presence of high quantities of water in the reaction mixture.
The invention also relates to the process for preparing phenol in which the first preparation step of cumene is effected by the alkylation of benzene according to what is specified above.
2. Description of the Background
Cumene is an important precursor for the production of phenol, which in turn is useful as an intermediate in the preparation of caprolactam from which nylon is produced.
The complete preparation process of phenol comprises the alkylation of benzene to cumene and the oxidation of cumene to the corresponding hydroperoxide which, by acid treatment, generates phenol and acetone.
With respect to the first alkylation step, catalysts based on phosphoric acid and infusorial earth in a fixed bed reactor or AlCl3 in slurry, are still widely used in the petrochemical industry, for alkylating benzene with propylene.
These processes however create problems relating to environmental impact and safety: in fact, the use of these catalysts is particularly problematical due to corrosion, the by-production of toxic organic products and disposal of the exhausted catalysts.
In 1965 the preparation of cumene using X or Y zeolite was described for the first time (Minachev, Kr. M., et al, Neftekhimiya 5 (1965) 676). The use of zeolites with a faujasitic structure for the alkylation of benzene with light olefins, such as propylene, was subsequently described by Venuto et al. in J. Catal.5, (1966)81.
U.S. Pat. No. 4,292,457 describes the use of ZSM-5 type zeolites for alkylating benzene with propylene.
Excellent results, in terms of industrial application, have been obtained in the synthesis of cumene using zeolites with a beta type structure, as described in EP 432,814, and in particular using catalysts comprising beta zeolite as described in EP 687,500.
Once obtained, cumene is transformed into phenol by means of an oxidation step to cumylhydroperoxide, followed by an acid treatment step which causes breakage of the peroxide bond with the formation of phenol and acetone.
If, on the one hand, the simultaneous production of phenol and acetone in a single productive unit is certainly a positive aspect from an industrial point of view, on the other hand the existence of an unbalanced commercial demand for the two products can cause problems in running an industrial plant for the production of phenol.
The necessity is therefore strongly felt for finding a possible use for present and future excess quantities of acetone.
U.S. Pat. No. 5,017,729 describes a process for the production of phenol via cumene hydroperoxide characterized by the use of propylene, in the preparation step of cumene, either totally or partially deriving from the reduction with hydrogen of acetone (co-produced with phenol) and subsequent dehydration of isopropylic alcohol.
The high costs of the various steps for re-obtaining pure propylenexe2x80x94to be used in the alkylation stepxe2x80x94starting from the acetone co-produced with phenol, are evident in this process.
In particular, in the process proposed by Mitsui for the production of propylene starting from acetone, it appears that the higher investment costs can be attributed to the dehydration section of isopropanolxe2x80x94obtained from acetone in the relative reduction section with hydrogenxe2x80x94to propylene.
It is known, in fact, that the dehydration step of isopropylic alcohol to propylene is necessary, for concrete industrial application, due to the impossibility of carrying out the alkylation of benzene directly with isopropylic alcohol as alkylating agent, when acid catalysts of the conventional type are used, as a result of the water released from IPA during the reaction, which has negative effects on the catalyst performance in terms of selectivity and, above all, duration of the catalyst itself.
Acid catalystsxe2x80x94both of the zeolitic and non-zeolitic typexe2x80x94are in fact negatively influenced by the presence of water which is developed when isopropylic alcohol is used as alkylating agent of benzene to give cumene.
In the case of a catalyst of the conventional type such as, for example, phosphoric acid supported on silica, widely used in the industrial synthesis of cumene, quantities of water higher than a few hundred ppm in the reaction mixture produce a significant chemical and mechanical disgregation of the catalyst together with a considerable lowering of catalytic performances in terms of yield to cumene.
In the case of catalysts based on zeolites, the negative effect due to the presence of water which causes a lowering of the overall yield to cumene, together with a more or less rapid deactivation of the catalyst itself, is known. All these negative effects are known and verified also with very low contents of waterxe2x80x94present in the reactionxe2x80x94with respect to those caused by the use of isopropylic alcohol as alkylating agent of benzene to give cumene in a process applicable on an industrial scale.
The industrial applicability of an alkylation process of benzene with isopropylic alcohol cannot, in fact, fail to take into account certain parameters such as for example the molar ratio benzene/IPA in the feeding to the reaction section, which generally ranges from 4 to 8 with a corresponding concentration of water in the reaction equal to about 48,300 and 26,000 ppm w/w, when the alkylation is carried out with isopropanol alone, assuming the total conversion of the isopropanol itself.
The possibility of using IPA as alkylating agent of benzene to give cumene is described in Appl. Cat. A 95 (1993) 53-63, whose disclosure specifies that the relative process must be rigorously carried out in gas phase, and yet there is still a significant loss in the activity of the zeolitic catalyst used as the test proceeds.
U.S. Pat. No. 5,015,786 describes a production process of phenol via cumene in which part of the cumene derives from the alkylation of benzene, also effected with isopropylic alcohol obtained by the reduction of the acetone co-produced with phenol, together with cumene deriving from the alkylation of benzene with propylene.
The alkylation step of benzene with IPA is carried out in the presence of a catalyst of an acidic nature, selected from various materials: zeolites are indicated as preferred catalysts. It is interesting to note however that the above document provides no information as to the life of the catalyst and constancy of the performances in general due to the fact that the test with the longest duration lasts 200 hours (Example 5, column 15) which correspond, under the conditions specified, to a productivity not higher than about 100 Kg of cumene/Kg of catalyst.
The avoid the problems mentioned above, the use of particular hydrophobic zeolites has been proposed, such as ZSM-5 zeolite with a high silica/alumina ratio or dealuminated H-mordenite and Y zeolite.
For example, in U.S. Pat. No. 5,160,497 a dealuminated Y zeolite is used, with a molar ratio SiO2/Al2O3 ranging from 8 to 70, for the alkylation of benzene with propylene and isopropanol.
We have now found that it is possible to obtain cumene by the alkylation of benzene with isopropylic alcohol, or a mixture of IPA and propylene as alkylating agent, by means of a process which provides better performances and above all catalyst duration, even in the presence of large quantities of water, using a zeolitic catalyst and operating under certain pressure and temperature conditions.
An object of the present invention, in fact, relates to a process for the alkylation of aromatic compounds in which the alkylation reaction is carried out with isopropanol, or mixtures of isopropanol with propylene and under such conditions that the quantity of water in liquid phase does not exceed 8,000 ppm w/w.